nano-particulate technology: synthesis. feynman’s vision in 1959 “there is plenty of room at the...

Nano-Particulate Technology: Synthesis

Feynman’s Vision in 1959

• “There is plenty of room at the bottom”– Microtechnology is a frontier to be pushed

back, like HP, HV, LT– Ordinary machines could build small

machines, which could build smaller machines,…. to atomic level

• 22 years later, first journal publication of article on molecular nanotechnology (Drexler, 1981)

Excerpts from “The Hindu” interview with Prof. Pradeep, Dept of Chemistry, IIT Madras; March 28, 2007

What is nano technology?

The term nano technology refers to a broad range of technologies, all of which involve the utilisation of the properties of nano scale objects. Nano scale refers to the size regime of nanometers or 10 to the power of -9 meters. The properties of materials in this size regime are unique. Nano technologies became possible as a result of our capability to manipulate matter with atomic precision. At the scale of nanometer, all disciplines converge. Therefore, nano technology is a fusion technology.

At this length scale, new properties and new phenomena come about. Materials start behaving differently. An example is reactive gold. Till now we knew only about noble metal gold, which does not change with time. Now we have highly reactive gold. In addition, we know of fluorescent and magnetic gold. This example suggests that numerous other materials with completely different properties could be made. This possibility is a result of the capability to manipulate matter at this length scale — the length scale of atoms.

Excerpts from “The Hindu” interview with Prof. Pradeep, Dept of Chemistry, IIT Madras; March 28, 2007

Why is it necessary to know about nano technology?

Well, look at nature. Leaves make complex molecules called carbohydrates starting from a single carbon molecule, carbon-dioxide, present in air. These molecules make life possible for all of us. Every molecular assembly in nature is by this atom-by-atom approach. From amoeba to elephant is made this way. These synthetic routes are the most energy efficient, green and sustainable. The motion of a muscle fibre, or a flagellum is the result of nano technologies. Therefore, ultimately an understanding of these will help us to do things better, with improved efficiency — in much more eco-friendly, sustainable manner. Of course when you look at properties at this length scale, one may find new things. That drives the other side of scientific enquiry — curiosity.

Nano-Engineered Products

• Semiconductor nano-crystallites for use in microelectronics

• Ceramics for use in demanding environments• Polymers with enhanced functional properties• Transparent coatings with UV/ IR absorption properties,

abrasion resistance• Static dissipative/ conductive films• Enhanced heat-transfer fluids• Catalysis• Topical personal care (e.g., sunscreen) &

pharmaceutical applications• Ultrafine polishing of e.g., rigid mememory disks, optical

lenses, etc.

Functional Polymer Fillers

• To improve viscoplastic properties• By addition of inorganic fillers

– Glass fiber, talcum, kaolin– 20-60% dosage– Disadvantage: incresed density of the composite

materials

• Late ’80s: Toyota developed nano-clays (“bentonite”) for automotive applications

• Functional polymers are very versatile, even tiny amounts can have dramatic impact

Other Applications• Nanowire & Nanotube arrays for EMI Shielding

– Superior thermal, electrical, mechanical properties– SWNT, MWNT– Metallic or semiconducting– Carbon nanotubes provide special advantage in shielding

• Chemical Gas Sensing– Low-power sensor arrays with high sensitivity, selectivity– e.g., humidity sensors, solid-state resistive sensors, combustible gas

sensors, etc.• Ceramic MEMS

– 2D & 3D microcomponents & microelectromechanical devices for harsh environments

• Energy Conversion:– Photo-voltaics, radiation detection, electroluminescent devices, etc.

• Electronics & Related Fields:– Scanning probe, scanning microscopy standards – Storage & memory media– Flat panel displays, etc.

Other Applications, cont’d• Marine Anti-Fouling:

– Nanoparticles held in coating lattice, not leached out by marine environment

– Slowly release ions to provide long-term protection– Assure longevity of antimicrobial activity

• Textile Fibers:– Nanoparticles in nylon, PP for antimicrobial character in extreme

environments, after extensive thermal cycling– Nanosized ZnO and CuO in synthetic fibers with minimal effects on

color & clarity• Permanent Coatings:

– For long-term antimicrobial protection in many coating formulations• Healthcare, insdustrial, food processing, general paints & coatings

• Catalysts:– Allows thinner active layers, less usage of precious metals– High, stable solids dispersions– Key application: automotive catalytic converters

Other Applications, cont’d

• Fuel Cells:– Rare-earth metal oxides , nanoceria– As components in electrodes– As low-temperature electrolytes in solid xide fuel cells

(SOFC)• Sunscreen:

– To protect human screen from harmful UV rays– Nanomaterials are effective sun blockers

• Semiconductor Polishing:– CMP slurries with fumed silica, collidal silica– Ceria, alumina dispersions in nano-sizes– High planarity, efficient removal, unique surface

chemistry

Nano-Particles

• Fundamental building blocks of nano-technology

• Starting point for “bottom-up” approaches for preparing nano-structured materials & devices

• Their synthesis is an important research component

Nano-Particle Synthesis Methods

• Colloidal processes– Bognolo, 2003

• Liquid-phase synthesis– Grieve et al., 2000

• Gas-phase synthesis– Kruis et al., 1998

• Vapor-phase synthesis– Swihart, 2003

• Sono-fragmentation– Gopi, 2007! (Ph.D. thesis)

Colloidal Process

• Nanoparticles produced directly to required specifications, assembled to perform a specific task

• Involves use of surface-active agents– e.g., CdS 50 nm particles by mixing two solutions

containing inverted micelles of sodium bis(2-ethyl hexyl) sulfosuccinate in heptane

– e.g., antiferromagnetic nanoparticles of Fe2O3 by decomposition of Fe(CO)5 in a mixture of decaline and oleyl sarcosine

• Coordinating ligands used to produce nanoclusters

• Surfactants play a major role

Vapor-Phase Synthesis• Vapor phase mixture rendered thermodynamically unstable relative to formation of

desired solid material– “supersaturated vapor”– “chemical supersaturation”– particles nucleate homogeneously if

• Degree of supersaturation is sufficient• Reaction/ condensation kinetics permit

• Once nucleation occurs, remaining supersaturation relieved by– Condensation, or– Reaction of vapor-phase molecules on resulting particles

• This initiates particle growth phase• Rapid quenching after nucleation prevents particle growth

– By removing source of supersaturation, or– By slowing the kinetics

• Coagulation rate proportional to square of number concentration– Weak dependence on particle size

• At high temperatures, particles coalesce (sinter) rather than coagulate– Spherical particles produced

• At low temperatures, loose agglomerates with open structures formed• At intermediate temperatures, partially-sintered, non-spherical particles form• Control of coagulation & coalescence critical• Nanoparticles in gas phase always agglomerate

– Loosely agglomerated particles can be re-dispersed with reasonable effort– Hard (partially sintered) agglomerates cannot be fully redispersed

Liquid-Phase Synthesis

• Used widely for preparation of “quantum dots” (semiconductor nanoparticles)

• “Sol-Gel” method used to synthesize glass, ceramic, and glasss-ceramic nanoparticles

• Dispersion can be stabilized indefinitely by capping particles with appropriate ligands

Sol-Gel Method

• Aqueous or alcohol-based• Involves use of molecular precursors, mainly alkoxides

– Alternatively, metal formates• Mixture stirred until gel forms• Gel is dried @ 100 C for 24 hours over a water bath,

then ground to a powder• Powder heated gradually (5 C/min), calcined in air @

500 – 1200 C for 2 hours• Allows mixing of precursors at molecular level

– better control• High purity• Low sintering temperature• High degree of homogeneity• Particularly suited to production of nano-sized multi-

component ceramic powders

Gas-Phase Synthesis

• Supersaturation achieved by vaporizing material into a background gas, then cooling the gas– Methods using solid precursors

• Inert Gas Condensation• Pulsed Laser Ablation• Spark Discharge Generation• Ion Sputtering

– Methods using liquid or vapor precursors• Chemical Vapor Synthesis• Spray Pyrolysis• Laser Pyrolysis/ Photochemical Synthesis• Thermal Plasma Synthesis• Flame Synthesis• Flame Spray Pyrolysis• Low-Temperature Reactive Synthesis

Inert Gas Condensation

• Suited for production of metal (e.g., Bi) nanoparticles– Reasonable evaporation rates at attainable

temperatures• Procedure:

– Heat solid to evaporate it into a BG gas– Mix vapor with a cold inert gas to reduce temperature– Include reactive gas (e.g., O2) in cold gas stream to

prepare compounds (e.g., oxides)• Cntrolled sintering after particle formation used

to prepare composite nanoparticles (e.g., PbS/ Ag, Si/In, Ge/In, Al/In, Al/Pb)

Pulsed Laser Ablation

• Use pulsed laser to vaporize a plume of material– Tightly confined, spatially & temporally

• Can generally only produce small amounts of nanoparticles

• But can vaporize materials that cannot be easily evaporated– e.g., synthesis of Si, MgO, titania, hydrogenated-

silicon nanoparticles

• Strong dependence of particle formation dynamics on BG gas

Spark Discharge Generation

• Charge electrodes made of metal to be vaporized in presence of inert BG gas until breakdown voltage is reached– Arc formed across electrodes vaporizes small amount of metal– e.g., Ni

• Produces very small amounts of nanoparticles– but in a reproducible manner

• Reactive BG gas (e.g., O2) can be used to make compounds (e.g., oxide)

• BG gas can be pulsed between electrodes as arc is initiated– Pulsed arc molecular beam deposition system

Ion Sputtering

• Sputter solid with beam of inert gas ions– e.g., magnetron sputtering of metal targets

• Low pressure (appr 1 mTorr) required– Further processing of nanoparticles in aerosol

form difficult

Chemical Vapor Synthesis• Vapor phase precursors brought into a hot-wall reactor under nucleating

condition– Vapor phase nucleation of particles favored over film deposition on surfaces– CVC reactor (Chemical Vapor Condensation) versus CVD

• Very flexible, can produce wide range of materials• Can take advantage of huge database of precursor chemistries developed

for CVD processes• Precursors can be S, L or G under ambient conditions

– but delivered to reactor as vapor (using bubbler, sublimator, etc)• Examples:

– Oxide-coated Si nanoparticles for high-density nonvolatile memory devices– W nanoparticles by decomposition of tungsten hexacarbonyl– Cu and CuxOy nanoparticles from copper lacetonate

• Allows formation of doped or multi-component nanoparticles by use of multiple precursors

– nanocrystalline europium doped yttria from organometallic yttrium & europium precursors

– erbium in Si nanoparticles– zirconia doped with alumina – one material encapsulated within another (e.g., metal in metal halide)

• Can prevent agglomeration

Spray Pyrolysis

• Use of a nebulizer to inject very small droplets of precursor solution

• Also known as aerosol decomposition synthesis, droplet-to-particle conversion

• Reaction takes place in solution in the droplets, followed by solvent evaporation

• e.g.: preparation of TiO2 and Cu nanoparticles

Laser Pyrolysis/ Photothermal Synthesis

• Precursors heated by absorption of laser energy

• Allows highly localized heating & rapid cooling

• Infrared (CO2) laser used– Energy absorbed by precursors, or by inert

photosensitizer (SF6)

• e.g.: Si from silane, MOS2, SiC• Pulsed laser shortens reaction time, allows

preparation of even smaller particles

Thermal Plasma Synthesis

• Inject precursors into a thermal plasma• Precursors generally decomposed fully

into atoms…• Which then react or condense to form

particles– When cooled by mixing with cool gas, or

expansion through a nozzle

• Used for production of SiC and TiC for nanophase hard coatings

Flame Synthesis

• Particle synthesis within a flame• Heat produced in-situ by combustion reactions• Most commercially successful approach

– Millions of metric tons per year of carbon black and metal oxides produced

• Complex process, difficult to control• Primarily useful for making oxides• Recent advances:

-Fe2O3 nanoparticles Titania, silica sintered agglomerates

• Application of DC electric field to flame can influence particle size

Flame Spray Pyrolysis

• Directly spray liquid precursor into flame

• Allows use of low-vapor-pressure precursors

• Applied to synthesis of silica particles from hexamethyldisiloxane

Low-Temperature Reactive Synthesis

• React vapor phase precursors directly w/o external addition of heat– and w/o significant production of heat

• e.g.: ZnSe nanoparticles from dimethylzinc-trimethylamine and hydrogen selenide– by mixing in a counter-flow jet reactor at RT– heat of reaction sufficient to allow particle

crystallization

Advances in Instrumentation for Nano-Particle Synthesis

Need to analyze processes with short time-scales, in small regions of a reactor, in complex mixtures

• FTIR spectroscopy (in emission & transmission modes) to simultaneously characterize – gas temperature, – gas concentrations,– particle temperature, and– particle concentration during synthesis

• Localized thermophoretic sampling and in-situ light scattering measurements of– particle concentration,– size, and– morphology

• Particle mass spectrometry and TEM imaging of extracted samples

Advances in Modeling for Nano-Particle Synthesis

• Compute particle nucleation rates based on detailed chemical reaction kinetics– in cases where nucleation does not occur by simple

condensation of a supersaturated vapor

• Model multi-dimensional particle size distributions– where both particle volume and surface area are

explicitly treated

• Model simultaneous coagulation and phase segregation in multi-component particles containing mutually immiscible phases

Sonochemical Nano-Synthesis

• Sonochemistry: molecules undergo a chemical reaction due to application of powerful ultrasound (20 kHz – 10 MHz)– Acoustic cavitation can break chemical bonds– “Hot Spot” theory: As bubble implodes, very high temperatures

( 5,000 – 25,000 K) are realized for a few nanoseconds; this is followed by very rapid cooling (1011 K/s)

– High cooling rate hinders product crystallization, hence amorphous nanoparticles are formed

• Superior process for:– Preparation of amorphous products (“cold quenching”)– Insertion of nano-materials into mesoporous materials

• By “acoustic streaming”– Deposition of nanoparticles on ceramic and polymeric surfaces– Formation of proteinacious micro- and nano-spheres

• Sonochemical spherization– Very small particles

Sonochemical Nano-Synthesis: Examples

• S-2, Se-2, Te-2

– used in non-linear optic detectors, photorefractive devices, photovoltaic solar cells, optical storage media

• Gold, Co, Fe, Pg, Ni, Au/Pd, Fe/Co• Nanophased oxides (titania, silica, ZnO, ZrO2, MnOx

– More uniform dispersion, higher surface area, better thermal stability, phase purity of nanocrystalline titania reported

• MgO coating on LiMn2O4

• Magnetic Fe2O3 particles embedded in MgB2 bulk• Nanotubes of C, hydrocarbon, TiO2, MeTe2

• Nanorods of Bi2S3, Sb2S3, Eu2O3, WS2, WO2, CdS, ZnS, PbS, Fe3O4

• Nanowires of Se

Sono-Processing of Nanocomposites

• Power ultrasound can assist in synthesis, blending, dispersion & erosion-testing of nano-composites– dispersed phase having at least one dimensin < 100

nm• High-intensity ultrasound used with melt

processing for polymer-clay nano-composites– e.g., PP/PS-clay & PMMA/clay nano-composites

prepared by ultrasonic-assisted melt mixing– clay aggregates more finely dispersed – Superior overall homogeneity of composite, improved

performance

Sono- Fragmentation(Size Reduction)

Particles

Bubble


Particles

Bubble

Bubble Collapse due to Implosion

Particle Fragments due to

a) Violent Bubble collapse

b) Inter-particle attrition


Particles

Bubble

Bubble Collapse due to Implosion

Particle Fragments due to

a) Violent Bubble collapse

b) Inter-particle attrition

Fragmented Particle

Feed Sample

Micron sized feed particles

Distilled Water

Sonication

20 kHz, 1000 W, Probe type Sonication/

58 kHz, 500 W, Tank


Distilled Water

Sono-Processed Sample

20 kHz, 1000 W, Probe type Sonication/

58 kHz, 500 W, Tank


Distilled Water

Sub-Micron /Nano Sized Particles

Micron sized particles

Sono-Processed Sample(stratified Mix)

Sub-micron/ nano Particles

Micron Sized Particles

Sono-Blending(Particle Size De-stratification)



High Frequency Sonication

Sono-Blended Particles For Composite Formulation



High Frequency Sonication

Good Blend of Sub-micron /Micron-sized particle

Drying out at 105 Deg C

Blended sample Ready for composite Formulation

Polymer Precursor Preparation


Solvent

e.g CHCl3

Polymer Precursor Preparation


Solvent

e.g CHCl3

Sonication For 2 mts

Polymer Precursor

( Particles Dispersed in solvent)

Polymer Matrix

Particle Reinforced Polymer Matrix

ParticlePolymer Matrix

Caviation Erosion On the ceramic Particle Reinforced Polymer Matrix

ParticlePolymer Matrix

Cavitation Bubble

Superior Cavitation Erosion Resistance on Nano-Composites

Erosion Resistance Enhancement, PMMA

0

0.3

0.6

0.9

1.2

0 2 4 6 8 10

Sonication Time (minutes)

Tu

rbid

ity

N.T

.U

Withfiller material

with out filler material

Erosion Resistance Ehancement

0

0.0005

0.001

0 2 4 6 8 10

Sonication time (minutes)

Ma

ss

lo

ss

(g

ra

ms

)

with filler material

without filler material

- Mass loss and turbidity data show same relative trends

-Sono-Cavitation test results shown to correlate with classical impact-erosion test results.

Thickness (m) VL VS

Density mu

LAMBDA E nu G

Unfilled PMMA 0.541 1.232 0.675 1.1 0.501 0.669 1.288 0.286 0.501

Filled PMMA 0.8712 1.421 0.846 1.16 0.83 0.683 2.034 0.226 0.83

E = Youngs Modulus in GPa.

G = Shear Modulus in GPa.

Nu = Possion’s ratio.

VL= longitudinal velocity mm/micro sec.

VS= Shear velocity mm/micro sec.

Lamda and mu are Lamis constant

WFA Filled PMMA has Higher E.moduli and shear moduli

Conclusion

• Nano-particulate technology is gaining prominence as nano-science becomes “old news” (& pico-science, femto-science begin to emerge!)

• Opportunities abound in scale-up & commercialization of nano-particle synthesis

• “Bottom-up” & “Top-down” methods are both used– pro’s & con’s must be weighed for specific application

• PSP Lab in ChE Dept @ IITM has cutting-edge research program in various aspects of nano-technology

nano-particulate technology: synthesis. feynman’s vision in 1959 “there is plenty of room at the...

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